1. Field of the Invention
The present invention relates to the particles for display media and the information display panel using the above particles, in which the display media having optical reflectance and charge characteristics are sealed between two substrates, at least one substrate being transparent, and, in which the display media, to which an electrostatic field is applied, are made to move so as to display information such as an image.
2. Description of Related Art
As an information display device substitutable for liquid crystal display (LCD), information display devices with the use of technology such as an electrophoresis method, an electro-chromic method, a thermal method, dichroic-particles-rotary method are proposed.
As for these information display devices, it is conceivable, as compared with LCD, as inexpensive visual display device of the next generation from a merit having wide field of vision close to normal printed matter, having smaller consumption, or having a memory function, and spreading out to a display for portable device and an electronic paper is expected. Recently, electrophoresis method is proposed that microencapsulate dispersion liquid made up with dispersion particles and coloration solution and dispose the liquid between faced substrates, and also it is expected.
However, in the electrophoresis method, there is a problem that a response rate is slow by the reason of viscosity resistance because the particles migrate among the electrophoresis solution. Further, there is a problem of lacking imaging repetition stability, because particles with high specific gravity of titanium oxide is scattered within solution of low specific gravity, it is easy to subside, difficult to maintain a stability of dispersion state. Even in the case of microencapsulating, cell size is diminished to a microcapsule level in order to make it hard to appear, however, an essential problem was not overcome at all.
Besides the electrophoresis method using behavior in the solution, recently, a method wherein electro-conductive particles and a charge transport layer are installed in a part of the substrate without using solution is proposed. [The Imaging Society of Japan “Japan Hardcopy '99” (Jul. 21-23, 1999) Transaction Pages 249-252] However, the structure becomes complicated because the charge transport layer and further a charge generation layer are to be arranged. In addition, it is difficult to constantly dissipate charges from the electro-conductive particles, and thus there is a drawback on the lack of stability during display rewriting.
As one method for overcoming the various problems mentioned above, an information display panel is known, in which the display media having optical reflectance and charge characteristics are sealed between two substrates, at least one substrate being transparent, and, in which the display media, to which an electrostatic field is applied, are made to move so as to display information such as an image. In the information display panel, in which the display media are moved by an electrostatic field so as to display information such as an image, in order to make easy for the display media to have positively or negatively chargeable properties and to maintain a charge amount, use is made of acrylic resin, methacrylic resin and styrene resin as materials of the particles for display media having a particle diameter of 0.1-50 μm. As a producing method, in order to make the process simple or to make the energy consumption low, or, in order to obtain directly the target particle diameter, it is effective to use a suspension polymerization method as compared with a crushing method. Moreover, when the particles for display media made of resin are produced by the suspension polymerization method, one functional monomer, which is dispersed as an oil drop in a suspended solution by using normally a surface activating agent, is polymerized to make it high molecular weight.
As mentioned above, when the particles for display media produced by using one functional monomer as the resin materials are used for the information display panel, there is a problem such that a display becomes impossible if it is exposed in a high temperature. The reasons are as follows. Since it is exposed in a high temperature, the particles for display media become a softened state even under a glass transition temperature Tg of its resin, and they are melted and bonded with each other to become a non-movable state. As a result, a display becomes impossible. Further, when the particles for display media are left as they are at room temperature after a display, there is a problem such that a voltage (drive voltage), which is necessary to move the particles for display media after the display, becomes high. The reasons are as follows. Since the resin produced by using one functional monomer have normally a thermal plasticity, the particles for display media produced by using the resin mentioned above have a creeping property even at room temperature. As a result, in the information display panel produced by using the particles for display media mentioned above, the movement of the particles for display media is prevented due to the bonding between the particles for display media, so that the voltage (drive voltage), which is necessary to move the particles for display media after leaving them as they are, becomes high.
As a method of solving the problems mentioned above, it is thought about a method such that a cross-linking monomer is used for example as a part of or all of molecule and use is made of the resin having a higher glass transition temperature Tg. However, when such a resin is used as the particles for display media having a predetermined intended use, a reverse driving property of the particles for display media is decreased if a rate of the cross-linking monomer in molecule becomes higher in some measure. As a result, a display property of information such as an image is decreased. This is empirically demonstrated. Therefore, in order to prevent the problems of a low thermal resistance and a creeping property in the particles for display media as mentioned above, and, in order to satisfy a display property of information such as an image, it is important to suppress a rate of the cross-linking monomer in molecule of the particles for display media within an appropriate range.